Over 200 million years ago, a mass extinction wiped out an estimated 76 percent of marine and terrestrial species. It marked the end of the Triassic period and the onset of the Jurassic, clearing the way for dinosaurs to dominate Earth for the next 135 million years.

It's not clear what caused the end-Triassic extinction, although most scientists agree on a likely scenario; over a relatively short time period, massive volcanic eruptions from a large region known as the Central Atlantic Magmatic Province (CAMP) spewed forth huge amounts of lava and gas, including carbon dioxide, sulfur and methane.

This sudden release of gases into the atmosphere may have created intense global warming, and acidification of the oceans, which ultimately killed off thousands of plant and animal species.

Now, researchers have determined that these eruptions occurred precisely when the extinction began, providing strong evidence that volcanic activity did indeed trigger the end-Triassic extinction. The scientists determined the age of basaltic lavas and other features found along the East Coast of the United States, as well as in Morocco--now-disparate regions that, 200 million years ago, were part of the supercontinent Pangaea.

The rift that ultimately separated these landmasses was also the site of CAMP's volcanic activity.

Today, the geology of both regions includes igneous rocks from the CAMP eruptions as well as sedimentary rocks that accumulated in an enormous lake. The researchers used a combination of techniques to date the rocks and to pinpoint CAMP's beginning and duration.

From its measurements, they reconstructed the region's volcanic activity 201 million years ago, discovering that the eruption of magma--along with carbon dioxide, sulfur and methane--occurred in repeated bursts over a period of 40,000 years, a short span in geologic time.

"This extinction happened at a geological instant in time," says co-author Sam Bowring, a geologist at MIT. "There's no question the extinction occurred at the same time as the first eruption."

The end-Triassic extinction is one of five major mass extinctions in the last 540 million years of Earth's history.

For several of these events, scientists have noted that large igneous provinces, which provide evidence of widespread volcanic activity, arose at about the same time.

But, as Bowring points out, "just because they happen to approximately coincide doesn't mean there's cause and effect."

For example, while massive lava flows overlapped with the extinction that wiped out the dinosaurs, scientists have linked that extinction to an asteroid collision.

"If you want to make the case that an eruption caused an extinction, you have to be able to show at the highest possible precision that the eruption and the extinction occurred at exactly the same time," Bowring says.

For the time of the end-Triassic, researchers have dated volcanic activity to right around the time fossils disappear from the geologic record, providing evidence that CAMP may have triggered the extinction. But these estimates have a margin of error of one to two million years. "A million years is forever when you're trying to make that link," Bowring says.

For example, it's thought that CAMP emitted a total of more than two million cubic kilometers of lava.

If that amount of lava were spewed over a period of one to two million years, it wouldn't have the same effect as if it were emitted over tens of thousands of years.

"The timescale over which the eruption occurred has a big effect," Bowring says.

Tilting toward extinction

To determine how long the volcanic eruptions lasted, the group combined two dating techniques: astrochronology and geochronology.

The former is a technique that links sedimentary layers in rocks to changes in the tilt of the Earth.

For decades, scientists have observed that the Earth's orientation changes in regular cycles as a result of gravitational forces exerted by neighboring planets.

The Earth's axis tilts at regular cycles, returning to its original tilt every 26,000 years. Such orbital variations change the amount of solar radiation reaching the Earth's surface, which in turn has an effect on the planet's climate, known as Milankovich cycles.

This cyclical change in climate can be seen in the types of sediments deposited in the Earth's crust.

Scientists can determine a rock's age by first identifying cyclical variations in deposition of sediments in quiet bodies of water, such as deep oceans or large lakes.

A cycle of sediment corresponds with a cycle of the Earth's tilt, established as a known period of years.

By seeing where a rock lies in those sedimentary layers, scientists can get a good idea of how old it is. To obtain precise estimates, researchers have developed mathematical models to determine the Earth's tilt over millions of years.

Bowring says the technique is good for directly dating rocks up to 35 million years old, but beyond that, it's unclear how reliable the technique is.

He and colleagues used astrochronology to estimate the age of the sedimentary rocks, then tested those estimates against high-precision dates from 200-million-year-old rocks in North America and Morocco.

The geologists broke apart rock samples to isolate tiny crystals known as zircons, which they analyzed to determine the ratio of uranium to lead.

The technique enabled the team to date the rocks to within approximately 30,000 years--a precise measurement in geologic terms.

Taken together, the geochronology and astrochronology techniques gave the geologists precise estimates for the onset of volcanism 200 million years ago.

The techniques revealed three bursts of magmatic activity over 40,000 years--a short period of time during which massive amounts of carbon dioxide and other gas emissions may have drastically altered Earth's climate.

While the evidence is the strongest thus far for linking volcanic activity with the end-Triassic extinction, Bowring says that more work can be done.

"The CAMP province extends from Nova Scotia all the way to Brazil and West Africa," he says. "I'm dying to know whether those are exactly the same age."